Pulse sequences. Categorization Spin echo Spin echo Conventional spin echoConventional spin echo...

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Transcript of Pulse sequences. Categorization Spin echo Spin echo Conventional spin echoConventional spin echo...

  • Pulse sequences

  • CategorizationSpin echoConventional spin echoFast spin echoInversion recoveryGradient echoCoherentIncoherentSteady state free precessionUltra-fast imaging

  • Conventional spin echoIllustrationThe situation before the patient is placed inside the magnet

  • The patient is now inside the bore of the magnet. The magnetization of the patients protons M0 is aligned along the Z axis

  • The 900 pulse is now applied. M0 is now completely removed from the Z axis and lies along the Y axis

  • Relaxation is now taking place. Spin-lattice (T1)relaxation caused the magnetization to re-grow along the Z axis. Spin-spin relaxation causes the magnetization vectors to dephase (move apart) while still in the X-Y plane

  • 1800 pulse is now given. All the vectors now point in the opposite direction. The magnetization vectors rephase (come together) in the X-Y plane. As they come together the echo is being formed

  • The magnetization is now completely rephased in the X-Y plane and points along the Y axis. This causes the full height of the echo. The actual MRI signal is taken here.

  • Relaxation continues to take place. The magnetic vectors again dephase in the X-Y plane while the regrowth along the Z axis continues

  • Complete relaxation has taken place. There is no net vector in the X-Y plane, and the magnetization is full grown along the Z axis. This is identical to the situation before the 900 pulse was applied

  • The coordinate system in relation to the magnet

  • Short TE and short TR gives T1 weighted image Use two RF rephasing pulses generating two spin echoes to produce T2 and proton density weightingFirst echo has short TE and long TR produce proton density weightingSecond echo has a long TE and a long TR produce T2 weighting

  • usesGold standard for most imagingMay be used for every examintionT1 images useful for demonstrating anatomy because high SNRWith contrast enhancement T1 images show pathologyT2 images also demonstrate pathologyDiseased tissues are generally more oedematous and/or vascular. They have increased water content and, have a high signal on T2 images

  • Parameters T1 weightingShort TE 10-20 msShort TR 300 600 msTypical scan time 4-6 minProton density/T2Short TE 20 ms/long TE 80 ms+Long TR 2000 ms+Typical scan time 7-15 min

  • AdvantagesGood image qualityVery versatileTrue T2 weighting sensitive to pathologyDisadvantagesScan times relatively long

  • Fast spin echoIn contrast to conventional spin echo, fast spin echo applies a train of 1800 pulses per TR and different phase encoding steps are used.Each 1800 pulse produce an echo (proton density & T2)This drastically reduce the scan timeThe number of 1800 pulses in the train called the turbo factor or train length

  • E.g. if in conventional SE 256 phase matrix and 1 NEX is used, the scan time is 256TRIn FSE if the turbo factor is 16, the scan time is 256TR/16 =16TR

  • Uses Useful in most clinical applicationsCentral nervous system, pelvis, musculoskeletal regionsNoteFat remains bright on T2Unless fat saturation techniques are usedMuscles appear darker in FSE images Artefacts from metal implants is significantly reduced

  • Parameters T1 weightingShort effective TE less than 20msShort TR 300 600 msTurbo factor 2-6Typical scan time 30s to 1 minT2 weightingLong effective TE 100 msLong TR 4000 ms+Turbo factor 8-20Typical scan time 2 min

  • Advantages & DisadvantagesReduced scan timeHigh resolution matrices and multiple NEX can be usedImage quality improvedIncrease T2 informationSome flow and motion effects increasedIncompatible with some imaging optionsFat bright on T2 Reduces magnetic susceptibility effect, so should not be used when haemorrhage is suspected

  • Inversion RecoveryStarts with a 1800 inversion pulse.This inverts NMV through 1800 into full saturation.When inverting pulse removed NMV begins to relax back to B0 A 900 excitation pulse is then applied at a time TI (Time from Inversion) from the 1800 inversion pulse

  • TI

  • TI determines the weighting & contrastShort TI gives T1 contrastLong TI gives proto density contrastAfter the 900 excitation pulse 1800 rephasing pulse is applied at a time TEThis produces the spin echoTR is the time between each 1800 inverting pulse

  • Uses conventionally used to produce heavily T1 weighted images to demonstrate anatomy & in contrast enhanced imagingNow more widely used in conjunction with fast spin echo to produce T2 weighted images

  • Parameters

  • STIR (short TI inversion recovery)Uses TI that corresponds to the time it takes fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat.As a result the signal from fat is nulled.Used to achieve suppression of fat in T1 weighted images.TI 150 175 ms

  • FLAIR (Fluid attenuated inversion Recovery)The signal from CSF is nulled by selecting a TI corresponding to the time of recovery of CSF from 180 to the transverse plane and there is no longitudinal magnetization present.Used to suppress signal from CSF in T2 weighted imagesTI - 1700 -2200 ms